Carbides

Carbides

compounds of carbon and electropositive elements, primarily metals and certain nonmetals. Carbides can be subdivided into three main groups according to the type of chemical bond: ionic (or saltlike), covalent, and metallic. Certain carbides belong to the group of nonstoichiometric compounds, which are solid substances of variable composition that do not conform to the laws of stoichiometry.

Ionic carbides. Ionic carbides are formed by strongly electropositive metals; they contain metal cations and carbon anions. The group includes acetylenides with [C≡C]2− anions, which can be represented as products of the substitution of metals for hydrogen in acetylene, C2HH2, and as methanides (products of the substitution of metals for hydrogen in methane, CH4). The carbides of alkaline metals (LiC2 and Na 2CC2), magnesium (MgC2), and alkaline-earth metals (CaC2 and SrC 2), and higher carbides of rare earths (YC2 and LaC 2) and actinides (ThC2) are acetylenides. As the ionization potential of the metal in this group decreases, there is an increase in the tendency to form “polycarbides” with complex anions from carbon atoms

Table 1.Properties of some ionic carbides

Crystal structure

Density (g/cm3)

Melting point (°)

Heat of formation (Kcal/mole)*

Specific Volume resistance (μ Ohm.cm)

*1 Kcal/mole = 4.19 KJ/mole

Li2C2 …

rhombic

1.30

—

14.2

—

Na2C2 …

hexagonal

1.60

800

−4.1

—

(decomposition)

K2C2 …

hexagonal

1.62

—

—

—

Mgc2 …

tetragonal

2.07

—

21±5

—

CaC2 …

tetragonal

2.21

2300

14.1+2.0

—

BaC2 …

tetragonal

3.72

2000

12.1±4.0

—

(decomposition)

LaC2 …

tetragonal

5.25

2360

38.0

45

CeC2 …

tetragonal

5.56

2290

—

60

Be2C …

Cubic

2.44

2400

28.0

1.1 x 106

Al4C3 …

rhombohedral

2.95

2100

49.5

—

(MeC8, MeCi6, and MeC24). These carbides have a graphite-like lattice, in which the metal atoms are arranged between layers of carbon atoms. Upon reaction with water or dilute acids, ionic carbides of the acetylenide type—such as calcium carbide— decompose with evolution of acetylene (or acetylene mixed with other hydrocarbons and sometimes with hydrogen). The carbides Cu2C2 and Ag 2C2 explode on impact, have low chemical stability, decompose readily, and oxidize upon heating. The carbides Be 2C and AI4C3, which hydrolyze readily, with liberation of methane (see Table 1), belong to the methanide group.

Covalent carbides. Covalent carbides, typical examples of which are silicon carbide, SiC, and boron carbide, B4C (more correctly Bi2C3), are distinguished by the strength of their interatomic bonds. They exhibit a high degree of hardness, chemical inertness, and heat resistance and are semiconductors. The structure of certain covalent carbides, such as SiC, is similar to that of diamond. Their crystal lattices are giant molecules.

Metallic carbides. Metallic carbides are usually constructed as interstitial phases of carbon atoms in cavities of the crystal lattices of transition metals. The nature of these carbides, as interstitial phases, determines their great hardness and wear resistance, the near absence of plasticity at ordinary temperatures, their brittleness, and the low level of other similar mechanical properties. The carbides of this group are good electric conductors (hence the name “metallic”). Many of them are superconductors (the temperatures of transition into the superconductor state are as follows: Nb2C, 9.18°K; NbC, 8°-10°K; Mo2C, 12.2°K; MoC, 6.5°K). The cross alloys of carbides TiC, ZrC, HfC, NbC, and TaC have properties that are valuable in technology. For example, compositions of 25 percent HfC and 75 percent TaC have the highest melting point (approximately 4000°C) of all refractory metals and substances. Metallic carbides have high chemical stability in acids and lower stability in alkalies. They form hydrocarbides, oxycarbides, and carboni-trides upon interaction with H2, 02, and N2, which are also interstitial phases and have properties similar to those of carbides. Compounds of more complex structure, such as Mn3C, Fe3C, C03C, and Ni3C (see Table 2), also belong to the metallic carbides.

Preparation and use. The most common methods used in the preparation of carbides include heating of mixtures of metal and carbon powders in an inert gas or reduced gas medium and fusion of metals, with simultaneous carbidization (MeO + C → MeC +CO) at temperatures of 1500°-2000°C. Methods used to manufacture products from carbide powders include powder metallurgy, decanting of molten carbides (usually under pressure in a gaseous medium to prevent decomposition at high temperatures), diffusion carburizing of products previously prepared from metals and nonmetals, precipitation as a result of a reaction in the gaseous phase (in particular during the production of carbide fibers), and plasma metallurgy. The mechanical methods generally used to process products made of metallic carbides and high-strength carbide-metal alloys have proved unsuitable and are being replaced by abrasive and ultrasonic processing and electric-spark techniques.

Among the ionic carbides, calcium carbide is important in technology as a valuable source of acetylene. Covalent and metallic

carbides have also come to be widely used. For example, refractory carbides are used in the manufacture of heating elements for resistance furnaces and protective casings for thermocouples and crucibles. Superhard and wear-resistant carbides serve as the base material for hard powdered metal alloys (tungsten-cobalt and titanium-tungsten) and for abrasives in grinding and finishing processes (in particular S i c and B4C). Carbides are a component of heat-resistant high-temperature alloys, (cermets), in which hard but brittle carbides are casehardened by viscous yet sufficiently refractory metals. Ferric carbide, Fe3C, forms a “cementite phase” in iron-carbon alloys (cast iron and steel) that is hard but extremely brittle and nonplastic. Because of their high chemical stability, carbides are used in chemical engineering and the chemical industry for the manufacture of pipelines, nozzles, and reactor linings. Because of their metallic or semiconductor conductance, good thermal emission properties, and the ability to be transformed into a superconducting state, they are used in the manufacture of resistors and various elements of semiconductor devices and in the construction of electric contacts, magnetic materials, and hot cathodes in electronics. (See Table 3 for mechanical properties of carbides.)

Eventually, carbide nanorods--rods on the nanometer scale--may serve as sensors, magnetic recording heads, or a component of superconducting materials that could raise their current-carrying capacity, Lieber says.

Excess Mn should not be reduced to a level at which the tensile strength-to-Brinell ratio decreases (hardness increases with no increase in tensile strength) and formation of FeS causes carbides and shows shortness or embrittlement problems.

use of appropriate grain size of molding materials, certain binders and moldable insulating materials were shown to decrease the tendency toward carbide formations in thin sections by decreasing the cooling rate.

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